yep. Looks like one of those bottles has been opened.
I have been buying tetra pond declor instead of aquariums stuff as it lasts longer and is cheaper in the long run - but have found myself throwing it away. Around 2 months after opening it it develops a stink. I recon its the contents breaking down once air has been allowed in (i.e. bottle opened and stored for length of time).
I throw it out when it gets like that. Simply didnt wanna risk using it.
I have since moved onto solid form doses of dechlor. Think the one Im usinf is called "safe" or something - can look it up if you like. It is a powder so will keep longer and dont need to add much - 100mg per 30 litres.