The bottles do not discharge into the engine but into the engine nacelle area.
The bottles are charged with Halon 1211 - a clear liquid-to-quickly-gaseous fire-extinguishing agent that leaves no residue once it's discharged into the nacelle. It's not Hollywood with white foam/powder/gas. It's neigh impossible to visually see any evidence of the extinguishing agent on surfaces after it has been discharged. There's discharge discs (part of your walkaround?) that disappear once the bottle's been discharged.
You bottles were probably charged and until your engineer accidentally discharged them - I doubt you were flying around with dead bottles.