First potential security-relevant incident observed on one Pixel 8 with GOS, and I'd be happy for a suggestion for next steps.

Here is what probably(*) happened:

0) GOS, secondary user
1) Using Vanadium (mostly out-of-the-box) in foreground, browsing.
2) Ordinary search for name of a journalist seen on TV (in this case: "Daniel Levy").
2.1) Done first with the search engine configured in Vanadium, in our case: startpage.com,
2.2) then continuing looking up the website of duckduckgo,
--> reaching a site that allegedly and probably actually was duckduckgo.com (no signs of irregular behavior jet)

2.3) Entering same search term there
--> getting an ordinary suggestion list of found sites

2.4) Probably(*) clicking 1 or 2 from upper search results, which didn't show any indicators of being shady.

(*) NOTE: I'm describing with restraint, because I'd expect some vagueness in memory here is possible because what happened next startled the one searching the internet.

3.) Suddenly a popup / overlay from "Download Manager" appeared and begin to enumerate unwanted names of files that were allegedly downloaded.
--> 4 files in total were supposedly downloaded until the next step.

NOTE: I am convinced, that no such download was intended, nor willfully initiated. I wouldn't even know how it was possible to start downloading multiple files at once in Vanadium.

4.) Person who operated the phone immediately shut down / rebooted GOS via side buttons > Reboot.

5.) Rebooting regularly > Unlocking Owner
6.) Auditor App > verifying Questionable Device (Owner) from other, trusted GOS phone
--> everything OK (green) and as expected

7.) Airplane Mode and Log into secondary user where the incident occured
8.) Auditor App > verifying Questionable Device (respective secondary user) from other, trusted GOS phone
--> everything OK (green) and as expected, too.

9.) Looking for the unwanted files via file manager (Files App)
--> Using the information from a another thread ( https://discuss.grapheneos.org/d/8701-download-manager-where-are-files-stored ), no obvious traces were found. Unfortunately, a notification history did not exist, because the feature is not enabled here.

10.) Using Files manager to dig through some typical folders mentioned in usage guide (s. https://grapheneos.org/usage#storage-access ) ,
--> nothing suspicious was found.
--> In this course, a few old, known files (jpg and pdf) were cleaned, but not our dubious, downloaded(?) files from step 3.)

11.) Opening Vanadium to find out more about the mysterious downloads:
12.) Download manager still showed file names:
_td.txt
_rd.txt
[1 undocumented txt file, unfortunately lost because suboptimal handling and/or the Download Manager's UI behaviour not fitting personal, mental logic]
_basicconfig.json

13.) Trying to find out more about the properties of these files(?)
--> mostly unsuccessful:
13.0) Few minutes of internet research does not tell me anything clearly useful.
13.1) In case of the alleged txt-Files, trying to obtain more info via Vanadium and Download manager.
--> Clicking the entries resulted in their successive disappearance from the Download Manager list - accompanied by a short, quick message bubble that probably read like: File not found.

13.2) The "_basicconfig.json"-entry actually did lead to an "Open File-Dialog", which was followed up by choosing BeauTyXT.
NOTE: I suppose steps 13.1) and 13.2) were risky. I am not proud about the handling here. Yet I want to be honest and as exact as memories allow it. Apart from that, there was (maybe too much??) trust in the presumed hardening of Vanadium and BeauTyXT and situational perceptions of the overall situational factors. That did not lead to thorough conviction that a dangerous attack was in progress. Actually, it's still unclear what exactly happened here, at all.
13.2.1) Using BeauTyXT to open the json(?)-file actually lead to something looking entirely like a relatively short configuration file in valid json, without many remarkable properties. I should have filmed the operation from another device or made a screenshot here, but I didn't expect that the file would soon get "lost"(?), too.
13.2.2) IIRC, BeauTyXT headlined the file with just its name and the locator "/", as if it was at top, root level of filesystem.
13.2.3) After I closed BeauTyXT, even that json(?)-File can no longer be found. I dont' know where it was(??) and where Download Manager put it or read it from.
14.) After step 13, the list in Vanadium > Downloads is now completely empty.

We could call it a day here, and pretend nothing happened, but an uneasy feeling remains. I want to emphasize that this is the first time something undesirable, unexplained happened on a GOS device, to me at all.

The least I'd hope to find out:

Where did these alleged files - of which I've definitely seen a json via BeauTyXT for a short time - go?
Is there anything to fear, clean up, or specifically watch for?
Generally, can anyone make sense of what have happened here?

Thanks in advance for your time and best regards!

PS: Excuse me for sketchy formatting. For the original post I did not have a preview, indentations came out completely different than expected, and now only few minutes to somehow make it tidy.