Chose a guy to go into waste and loaded him up with max packs, before he had left the vault I grabbed him by accident and put him in the living quarters with a female, when I realised he was in wrong place I dragged him outside again and the load up screen for Stims and meds came up again but everything I had assigned him was gone and had not been put back in the inventory of the vault.

Odd.. Did you check if maybe another dweller went outside with the stimpacks you supplied? Happened to me a couple times...

No other dwellers going out, this was my 1st