What about the obvious? The RC with the device. I presume you have it in your other hand (or suspended around your neck) when you are retrieving the P3?
Try moving the RC/device near the P3 and see if it affects it. Some devices have fairly strong magnets in the speakers. May or may not be the problem but its easy to check.
Headphones/earbuds? Those too have magnets. Wristwatch maybe?
I rarely experience any compass notifications. Only times is when I have taken off/landed on my truck's hard cover on the bed (due to the truck, not the cover).
I do not compass cal unless I have moved from the last one a
substantial distance. Across town is not substantial in my world. Next town over is not substantial.
I cal'd up here in Milwaukee last Feb was the most recent. Numerous flights since them I did another down in Indiana when I flew near a wind farm, obvious long way away from Wisconsin. I forgot to do one in Alabama (ALA-TN border) when I flew at a rest stop to video the Saturn rocket there even though last cal was in Indiana, I received no errors and multiple flights went just fine even though the compass hadn't been done since Indiana. And I did another cal in Florida and flew quite a bit around the panhandle off the one cal. And of course, another when I got back here to Milwaukee.
I haven't done an IMU cal either since winter (so I got a good cal in 7f outdoors). Instant on, no warm up delay