Entry sensor IS responding, but doesn't always trip

This is a Gen 3 system, installed and working for 4+ years. I have an entry sensor on the entrance door I usually use that started acting oddly several weeks ago. When the system is activated ("Away") and I enter through that door, it doesn't always detect it. That is, it senses that I entered about half the time. Some more info:

• I never get a "sensor not responding" error.
• When I test the sensor, it always works.
• When I open/close the door, it always flashes the blue light on the sensor.
• The battery was only a few months old when this started, but I replaced it again anyway. That did not help.
• It is the farthest sensor from the base station, so I tried swapping it with another sensor to see if the "new" one would have the same issue. It doesn't, so it seems to be the sensor itself that has a problem. (And anyway, if distance were the problem, I'd expect to get "not responding" errors, which I never do.)
• When I arm the system to leave the house, I always get the audible warning that the door is open, so I know the base station is communicating with the sensor and the sensor is correctly indicating that the door is open.

Very strange. Any idea what could be happening? I'm suspecting it's an intermittent fault internal to the sensor, so I should probably just replace it. But I first want to be sure I'm not overlooking anything. Besides, it seems so unlikely that it would fail so often in operation, but has never once failed when I test it.