I just noticed that the black triangular piece near my passenger side rear view mirror is loose towards the top. Is there a quick fix for this or should I get this serviced? I'm not sure if there is anything important within or behind this piece that I should be cautious of. The same piece on the driver's side is still completely intact.
This video explains the disassembly process and at around 2:30 he explains there is a clip that has to be installed going down or it would stick out (like yours is I guess).
I searched ebay for the mirror part, that shows that top clip better, which looks like a hook:
2017-2020 Tesla Model 3 Factory Left Door Mirror Chrome OEM 1110777-99-H | eBay
It doesn't look like it should easily pop out on its own. Only thing I would worry is if that clip on top is broken already or if you break it while trying to get it back in (wouldn't want to replace the whole mirror just for that little clip).
To properly remove the mirror requires taking out the whole door panel (in order to access the bottom 2 bolts and disconnect the wires), but you might be able to get a better view of the clip if you take out the speaker grill (5:50 in this video shows it). Although from the videos, it looks a like the rubber gasket on the inside covers up where the clip hooks (so can't really see how it hooks). Might also be able to loosen the top bolt at least to perhaps be able to have more wiggle room to get the clip back in. Again, I would be careful not to break things (from the disassembly videos it looks like the clip just hooks in from the top).
Another video that shows the process in even more detail: