They disappear. Can't remember which rank but it was around 1200 that I realized I hadn't seen one in awhile.

Stryll Seeker (Uses Left: 841)
I use them sparingly since I don't get any more.

ive got significantly less, under 300, but ive got another ~350 ranks to gain more, ill just have to really plan my scan runs to maximized for efficiency and not be on anyones newsfeed.. might even have to use the dreaded halc and calming amp traps ick