Wife misused blank signed cheques and filed cheque bounce 138 case?

When you say that you had 'lost' your office bag containing the said cheques, and your wife put them in the bank i.e. had the possession of the said cheques, are you stating that the 'lost' office bag was actually 'stolen' by your wife?

Let's sort your question regarding your defence to the cheque bounce case filed against you by your wife first. Lost / Stolen cheques which were duly informed by you to the bank for a stop payment and also informed by you to your wife, is a valid and strong enough defence under Section 138, Negotiable Instruments Act in addition to the defence that arises by the fact that there would be a difference in the handwriting / ink due to the cheques being blank and the same adds potency to your story of the cheques being lost / stolen.

Coming to look at this from another point of view, in case the lost cheques were infact in the possession of your wife, it amounts to theft, cheating and criminal breach of trust wherein you may file a complaint against your wife under Section 406 & 420 IPC amongst other applicable sections and charges.

Lastly, would conclude stating that all the above defence & rebuttals would hold ground provided you haven't entered into an agreement with your wife for the maintenance of your son for which you had not provided her the same through any mode (cash / cheque) and she thereby used the alleged 'lost' cheques to fulfil your obligation towards the said maintenance.

This is being said as by default, issuing a cheque means you have a financial obligation towards the holder / name bearer of the cheque unless proven otherwise by the defendant, i.e. the person issuing the cheque.