Well there’s always a fingertip magnet, which is pretty standard.
As far as non-standard install locations, I have had implants: between the 4th and 5th metacarpals, near the meaty base of the thumb, where a watch would go, near the elbow, in the upper arm, in the tragus near the ear.
I know ragebolus and leumas95 have some in their proximal phalange. Rosco’s got some in his foot and lower back. Plenty of people get the temp sensing implants in their torso.