Suspect:
SBS6600E - (BN SP > BN 18)
SBS8348X - (BN 18 > BN SP)
Been svc 18 daily for more than a month already. Meanwhile the
bus that is out should be SBS8348X, as I haven't spotted it on 18
ever since. Need someone to confirm.
I believe you didn't spot Sv 18 everyday did you? 6600E was on
Sv 40 for 2 days last week. Yes, 8348X is MIA from Sv 18 for a long
time (and for that matter 8321Y since it was 8321Y's slot in the
first place). But that missing slot has been done by random KUBs
& Citaros for the past weeks.
So no, 6600E is not on Sv 18 everyday like you claimed. Nor is
there any persistent cameo that I have seen so far on 8348X's (or
8321Y's) slot for the past weeks.
I don't rule out 8348X being out to SP alr given it was a temp
replacement for 8321Y anyway (which is probably in BNDEP for
overhaul or such like 8888D last time), but I think 8321Y should be
back on Sv 18 soon. Either way, just go observe more also then see
the trend. 
