I made some test in a machine when I noticed a strange interaction on the camera. No magnetic field was present.
I put some Thoron for the test. This indicate that the observed tracks are not alpha because they are very long. This is protons, and 2 tracks are easily observable from this event. But... if you pause at the beginning, there is another track, coming from the same origin. Unfortunately the origin is black, there is not enough vapor to reveal the interaction. Why ? because some seconds before, this location was also the origin of 2 big alpha track which consumed a huge amount of vapor. So for this particuliar event, the origin is not known. But we observe surely 3 protons tracks.

What it could be : Probably a neutron which ejected 3 protons from a nucleus (Carbon, Oxygen, Nitrogen..). We don't see the recoiling nucleus, because it's path is only a few milimeter long and in the black area.
Other explanation : Deviation of an incoming proton. This event is unlikely due to the direction of delta ray (electrons ejected during the path of the protons). And we don't observe a substantial energy loss after the interaction (the density of tracks are the same). And the direction of the third particle (the ghost one) don't fit with this explanation.

More information about the physics here :
https://www.cloudylabs.fr/wp/decays-o...