Denis,
MC92xxx are counted by the BTS, not by the BSC ! (new B10 counters…)
IE Name Request Reference = Random Access Reference = 19
Trace during night is a good idea.
If cell deep into the LAC, then I would assume those subscribers are could not connect to their operators and then they try to attach to your network in order to establish a call.
Check the IMSI/TMSI of those MS !
Regards
pix