I've had cracks round the motor mounts on my elderly P3P for around 4 years, but they don't appear to have got any worse UNTIL a recent close inspection with flexing the rotor arms by hand revealed this crack beside the shell screws. The photos are of the same arm, the second one is with the arm flexed. This is an alarming development, and will need to be repaired. I'll pop off the top shell, (YouTube is useful here) remove the motors and use Plast-aid, working from the inside and forming a 'puddle' of well diluted Plast -aid around the cracks, flexing the arms while it's still liquid to ensure it works into the cracks. IMHO, Plast-aid has to be better than superglue and baking soda, as it actually fuses into the shell material. Both the rear arms are in the same condition, so I'll repair them all.
This way I don't need to completely dismantle the P3, but I do get a robust and neat repair. I'll let you know how I get on..