I used to work for DS&S on the who do a lot of EHM for R-R & we monitored all sorts for the Trents, RB211, AE3007, V2500, BR715's etc.
The majority of alerts we issued for high N1 vibration, bleed leaks 'n' stuff. Bleed leaks we pretty easy to spot when you get a high spike in TGT, FF & dips in TGT margin. All engines have their little quirks as well.
I know for a fact it saved R-R a fortune on the Trent 500.