Stagecoach's Aldershot depot still has a fair number of ALX400-bodied Tridents which are primarily used on the 4xx Farnborough Colleges and All Hallows School routes, however these are interworked with various daytime routes during term time. Daytime public routes that are pretty guaranteed to turn out primarily ALX400s during weekday term term time are the 9 (Farnborough circular), 10 (Farnborough to Church Crookham), and one of the two vehicles on the 41 (Frimley Park Hospital to Ash). The latter is generally the 0930 Ash-FPH, 1035 FPH-Ash, 1120 Ash-FPH and 1225 FPH-Ash duty. Therefore I'd say Farnborough is a good place to see a good variety, as you'll see all these three routes there as well as some on long layover been school runs in the main road laybys directly opposite the main Kingsmead bus stops.
All three routes fall within the Blackwater Valley DayRider area and, for reference, Farnborough is around a 40-50 minute train journey from Waterloo (on the South Western Railway line out through Surbiton).
Tridents also make frequent sightings on the 65 between Guildford and Alton between school runs on the St Peter's School PTx routes (so 1025, 1125 and 1225 off Guildford and 1120, 1220 and 1320 return trips from Alton). The KITE between Guildford and Aldershot is another place they pop up quite a lot. Guildford is around 35 minutes from Waterloo, also on South Western Railway.
They still have at least one - I saw it today parked up between duties near the M25.
There are currently three left in the fleet - TA324 (SN03 EAE), TA717 (SN53 KHR) and TA950 (LX53 JYO). The older Darts and Tridents are gradually being replaced by Enviros retrofitted to Euro VI though, as a lot of their rail work now falls within the tightened up Low Emission Zone, but the Tridents are currently still hanging on on school routes and some rail. The sighting by the M25 was likely - if at the DVSA layby near Junction 9 - between school runs on the St Andrew's routes. Rather than run three buses dead back to depot in the morning and then out again in the afternoon, to save fuel and wear/tear at least one bus is generally parked there and the two drivers come back in one bus together (and the reverse in the afternoon).