...it could well be that there are ten times as many anonymous attacks...

No, it's not quite that bad! It's more like twice as many:

"Since the emergence of modern terrorism in 1968, 64% of worldwide terrorist attacks have been carried out by unknown perpetrators. Anonymous terrorism has been rising, with 3 out of 4 attacks going unclaimed since September 11, 2001. Anonymous terrorism is particularly prevalent in Iraq, where the US military has struggled to determine whether the violence was perpetrated by Shiite or Sunni groups with vastly different political platforms."

Abrahms references his analysis of a RAND dataset, and also Bruce Hoffman's "Why Terrorists Don't Claim Credit" (in Terrorism and Political Violence, Vol 9 #1 1997). I haven't read the latter, but his analysis seems enough for me.

I think there's definitely something of a mental bias here - it's vastly easier to remember the rare dramatic attack (which sooner or later someone will claim credit for) than the many anonymous ones.