Ah, it was largely a guess based on what I found for P0 or PO. You measured the lower right pin (pin 2) as 12V, and that's a thick trace so is likely the power input. Pin 1 has a thin trace so will be the control signal, which leaves pin 3 as the output. That then probably goes to the +ve pin on the fan header (middle pin), through the fan motor, down to Ground (fan header pin 1). So when that device is conducting both the input (pin 2, 12V supply) and the output (pin 3 to fan header) will be at around 12V. When it's not conducting then pin 3 will go to 0V.
An NPN transistor (like the PDTC124E) will conduct when the base (pin 1) voltage is above the emitter (pin 2), which would mean the base would have to be more than 12V, which seems very unlikely. Plus, you measured pin 2 as 12V, so current would have to flow from the emitter to the collector, which is wrong for an NPN. So it looks like we're switch on the high side of the fan, not the low side. So it's either a PNP BJT or P-ch FET.
The SO2894 is a PNP transistor, so that seems right for where this transistor is in the circuit. Current flowing from the Emitter (pin 2) to the Base (pin 1) allows current to flow from Emitter to Collector (pin 3). But it has a maximum Vce of 12V, which is exactly what the voltage be if the fan is off, and a maximum current of 200mA which doesn't seem high enough, and it's unlikely for the designer to have chosen a part to run at its absolute maximum.
That TP0101T has a max Vds of 20V and Id of ~500mA, which seems more likely. Plus, by the time this board was designed (98/99?) FETs would have been common for switching.
So, my guess is a P-ch FET, with the marking being type PO, week code 6 (so week 46-50 according to Vishay's week code table), lot 94.
Could be wrong though. You can check (with board off) that pin 3 does connect to pin 2 of the header. That'll confirm if Q7 is P and not N type. Also if you remove Q7 you can measure (with the board on) the voltage at pin 1 when the fan is supposed to be on, and when it's in power saving mode and the fan off. Also, measure the pin 1 voltages on the other fan control transistors. A difference between BJT and FET is that, for p type, the Emitter-Base voltage of a BJT won't be very large (just a diode drop, so <1V), so pin 1 will be around 11V. But a FET can have a large Source-Gate voltage (in this case probably 6V) so will be around 6V.