It seems the have taken up residence inside a hydroponic tray. The multi tool isn't working when I'm inside. How do I evict this squatter?

As far as I know, the sentinels spawn at random locations when a planet is generated. Occasionally they do spawn in unlikely places. I've had them crusing around inside my "greenhouse" several times. They don't seem to do anything, just bump into the planters once in a while. Wait until you have one spawn inside the roof structure of a round room; they just circle around and around and all you can see is the glow of their searchlight.

After you leave and respawn, they get regenerated. No sentinel traps required...