Indeed, much like a criminal defendant pleading the 5th is not proof of guilt. However, in the mind of the jury - and permissible under the law - is the inference that the defendant has something to hide.
In this case, the enemy having and/or wearing chemical suits while unloading drums is not "proof" per se - however, the inference remains and is equally permissible.
Ahh, now you bring in the Chemical Drums! In this case - it IS indeed suspicious. However, this all got started because of a Beeb article with the Headline "'Proof' of WMD's found". In the article was simply them mention of Chem Suit and atropine.