Like anything else in life, CYA is the name of the game! If you've ever been short changed at the store and caught it by counting your change before you walk away, you'll know what I mean. Whose to say your parts replenishment was correct? Whose to say that some desk weenie in data input forgot to enter the part on line 7 that was used off your van or worse the whole ticket got lost in the shuffle and ALL the parts you used on that ticket are still showing as unused and still on your van. Then your review comes around along with the required parts inventory check and you come up $800 short on parts. What do you think your review is going to look like. I repeat, CYA is the name of the game!

buff,
Not tracking your van stock, is like not tracking your in hours parts stock - you are open to "shrinkage" - inventory lost & not paid for, certain techs have been know to do "side jobs" at your expense, van use, fuel, parts & labor (unless the van goes home with the tech & work might be done on weekends or after hours. Been do before, been done today, will be done in the future.
Also, periodic review of open work orders needs to be done - especially those where a lot of parts have been charged to but the work order is never closed. Another area where parts can be charged to but never paid for.

Just sharing some past experiences in the US, probably never happened north of the border - see the US has stolen Lord Stanley's Cup once again.
A life long Red Wings fan.

Toyota / BT in the UK:

A technician's van is treated as a mobile workshop & any stock the technician orders for a repair / service or carries as stock will be on there work pda until its booked out to a job.

If a technician needs to do a stock transfer to another technician they have to ring the service desk & give there details plus the details of the technician the stock is being transfered to, service desk will check this with the technician receiving the stock & then do the transfer on the computer system.

Technicians can keep fast moving stock as replen parts & any of these booked out will be replaced on the nightly parts delivery drop of.

Stock checks are carried out roughly every 6 months at random by managment, all technicians parts stock be it based on site or mobile van is visable to all other service personal on the Toyota system.