Hi Emil
I hope you got on well.
What tends to happen so often is that there is an ingress of water. The client tries to dry it on a DIY basis and succeeds in drying the top of the fibres. The problem of the odour is coming from below where you would clean.
Of course a lot of the answers you get need to be filtered through what you actually find. Half the battle is getting an honest explanation as to what actually occurred, how much and what type of water flooded the carpet and what, if at all, the client has tried to do to remedy the situation.
As you have stated if it is an office carpet it is likely to be carpet tiles. There may well be moisture trapped below the bitchumen level of the tile. Uplifting and drying is the order of the day. it is also possible that the flooding has reacted with the glue sticking the tiles to the floor. This is much more problematical and there are no quick/cheap fixes. Explaining the situation to the client and setting out your stall (you do know what to do , don't you?) and charging accordingly is the way to put this problem to bed.
You must of course explain that whilst treating an odour you will end up masking the problem and as to whether you are successful will only be known once the treated area has dried out.
Explanations before you start doing anything are fine. If you wait until after you have worked on it will only sound like an excuse.
Good luck (and happy, steep learning curve).
Rog