When old bills are sent in to be destroyed, are the serial numbers recorded?
According to the gubmint websites, $20 bills last about 7.9 years, and "approximately 24.8 million notes a day with a face value of approximately $560 million" are printed (various denominations). Without attempting to estimate and do a bunch of math, at this glance I'll take a stab at it and say, such a recording of undamaged serial numbers might take place using current technology, but in the early 1970s, even with a lower volume of printing but a similar avg lifespan, it didn't. And since we're talkin' gov't operation, even if the serial numbers get recorded, there's probably no cross reference between those and the "fugitive" bills.
Of course they are. The money never entered circulation or it would have been detected long ago.