Electrical – Red wire with 40 Volts

I have a switch that I believe controls the lower socket on a split outlet. There is a red wire from the lower socket that goes into the switch, but it appears to be carrying 40 Volts AC relative to ground, and 50 Volts AC relative to live.

I have previously replaced the other switch in this gang, which is when I noticed that there was something odd. I replaced this switch too, and probably didn't wire it correctly (I didn't know about split outlets back then). The other switch in this gang controls overhead lights. There is a nearby gang of switches: one controls a fan, and I'm not sure what the other controls, but it has a red wire as well, so I suspect it's a different split outlet somewhere else. It's possible I've replaced an outlet somewhere and didn't notice that it was supposed to be split.

I'd appreciate advice on how I could possibly be getting 40 Volts on this red wire (should be either zero or 120, right?), and how it should be wired properly.

The switch is on the left in this photo: the red wire goes straight out of the junction box. The white wire from that switch is live (whoops on the colors).

The socket (prior to replacement, but I wired the new one the same way except I used the screws instead of backstabs) is seen in this photo. I have verified that there is continuity between the red wire in the socket and the red wire in the switch. There's a single neutral for both the upper and lower socket.