@MainDude
It would be useful to know where you were taking off from, your primary HF frequency, and at what time UTC. The general area would do. More likely you received the SELCAL co-assigned to another aircraft presumably operating a semi-hemisphere away and transmitted by a similarly remote HF station - more likely around sunrise / sunset.
With the available letters, there are somewhat less than 11k of unique SELCALs.
On the other-hand, how often have you been SELCAL'd in error!
mm43