Odd, "inverted" key/note on rev 3 Prophet 5 - any tech types able to help?

[Manbird]

Hi all - we've got a rev 3 P5 that was recently serviced... 6 months back-ish. It's been running fine until today when one note, the "E" in the middle of the keyboard, seems to have got stuck in "on" mode. The only way to stop it sounding is, ironically, to hold the key down. As soon as the key is released, though, the note sounds. Any guess as to what might cause such a problem? We're hoping it might be a simple fix, something we could even attempt at home, as our tech is a 2-hour drive from here.
Thanks...