Sounds to me like a stalling technique by the welders. Are they paid hourly rates? Production bonus?
At the start of the week I would issue all the fasteners required for each welder. I would then have them turn them in at the end of the week and dock them $1 for every missing item. Times are tough, welders are in good supply. You do not have to put up with crap like that when you are part of a business trying to make a buck to pay their wages.
Before doing that put in some covert cameras and watch what is happening to the bolts. They are probable tossing them away. Been there done that when I worked for an Engineering company in New Zealand. The welders were burning sticks down to half way then tossing them under machinery, in trash and anywhere they could find.
That meant they were using almost twice as many sticks per weldment and had to go to the supply cabinet and get more packs all the time. Fired a couple and then charged the rest for overs each Friday. Got their attention really fast.