You rejected for a selcal call? I could understand if you had someone knocking on your window, but not a selcal call!
All you need is those 4 tones coming consecutively on the HF to set the thing off. They were either strange sounds on the HF, or a mistaken code transmitted, or a code misidentified by your HF box. They were not a prank- I doubt whether anyone is daft enough to transmit various codes on the HF in the hope someone, somewhere might think it was a call for them.