In the English Grammar Profile, B2 point 67 in the category of PAST is defined as:
past perfect continuous after a time conjunction to give background information.
For example:
This was just a few months after Ebola had been terrifying the world.
1 AFTER IT HAD BEEN SITTING 46
2 AFTER HE HAD BEEN DRINKING 29
3 AFTER I HAD BEEN USING 29
4 AFTER IT HAD BEEN RUNNING 28
5 AFTER I HAD BEEN WORKING 19
6 AFTER HE HAD BEEN MISSING 18
7 AFTER WE HAD BEEN DATING 16
8 AFTER I HAD BEEN PLAYING 14
9 AFTER I HAD BEEN TAKING 14
10 AFTER WE HAD BEEN TALKING 13
11 AFTER HE HAD BEEN WORKING 12
12 AFTER I HAD BEEN DRIVING 12
13 AFTER THEY HAD BEEN DRINKING 11
14 AFTER I HAD BEEN WAITING 10
15 AFTER SHE HAD BEEN MISSING 10
16 AFTER I HAD BEEN SITTING 9
17 AFTER IT HAD BEEN RAINING 9
18 AFTER I HAD BEEN LIVING 8
19 AFTER I HAD BEEN MAKING 8
20 AFTER SHE HAD BEEN DRINKING 8