This alarm is very common on older machines. I expect that you have a faulty proximity switch cable to the shifter extend and retract switches. As the shifter sweeps from waiting pot to magazine or vice versa, the o/p vanishes momentarily, you will not see this on the I/O page, it is too fast. I always change both proximity switches as they normally have both been there the same length of time. Do not be tempted to cut the wire and put a join in, this will come back to haunt you many times, it is only a short run to the terminal box.
Note, the retracted switch has to be a short one, e.g 33mm long or you cannot reassemble the guards, but the extend one can be a 50mm long.
This normally sorts the problem, but if it occurs after the arm has retracted, also check the extend switch operator, this rod can get very stiff and allow both extend and retract to be on togethor, when the spring finally pulls it back, the arm will carry on normally to the parked position with the same alarm.
Whilst up the top, it is worth cleaning the tool in magazine optical switch, when gunged up, the system always sees no tool in magazine, not normally a problem unless you manually attempt to put a tool away after a hang up, in which case you can end up attempting to put one tool in the same pot as another. They do not normally bounce to well!!