Smuj,
As you surmised, back in the day, the intake blanks weren't fitted as often as they should have been. They weren't in the best condition either.
In our case, the 'jaspers' entered via a poorly fitting (not poorly fitted) intake blank.
Years later, while on a trip to Johannesburg, I got talking to a local apiarist. He assured me that the average swarm of African bees can construct a large comb in about 12 hours.
When we did the compressor wash in Nairobi, we first briefed ATC and asked permission. The a/c was parked facing away from, and quite close to, the Tower. When the crushed walnuts dropped from the hopper and travelled through the engine, cleaning the compressor blades, a rather large dark cloud emanated from the exhaust.
ATC came on the radio saying 'ASCOT XXXXX, what is it that you are doing? Everything is black here.'
We, of course, apologised.
You may, also, remember the incident a few years later where an engine was started with a blank still in the intake.
IIRC the a/c had come from the hangar and the blank was lying flat in the intake and, therefore, couldn't be seen from the ground.
The 'powers-that-be' tried to finger the crew - in particular the Air Eng - but their accusation fell apart when it was discovered that there were four blanks in their correct stowage in the cargo compartment and that the offending blank was, in fact, an extra one.
The external walk-round check was amended to reflect the fact that inspection of the intakes is restricted from the ground.
Another fine example of Sod's Law.