I might have found my own answer. Maybe other people can confirm or deny this explanation.
In the last hour I deployed and recovered my sensor somewhere around 20 times, and then at the very end I had the same problem as last night: I got the message saying I could press my Reload key to pick it up, but no amount of pressing Reload picked it up.
So it seems to be a glitch rather than planned behavior, and if so I was just unlucky that the first time I tried to recover my T-UGS the glitch was in effect.
The thing that both glitched situations had in common was that I'd deployed the sensor in an extremely tight area. The first time it was in the corner of a tight hallway and a Support player was prone right next to it with his ammo kit deployed.The second time it was in a wooded area and I just barely found a place that would accept the T-UGS.
So my working theory is that very tight deployments can sometimes trigger the glitch. They don't always trigger it, though, since I was able to recover a couple of other sensors deployed in similarly tight areas.
Google doesn't turn up discussion of this glitch, so I'll post on the official forum to see whether others can confirm.