Searching collapsed groups works by expanding them as they are

traversed by the search function. To mitigate leaving behind

expanded groups that were previously collapsed by the user,

the groups that were expanded by the search are flagged as

such so that they can be collapsed back automatically.
The search is started by typing while the Buddy List 

window is active, or by pressing CTRL+F in the Buddy List. 

Use the UP and DOWN arrow keys to move to the previous and 

next matches.

Tested on Debian Sid and Cinnamon DE