Smoking in the cockpit - how is this tracked?
Yes, they would be breathing second hand smoke, but aircraft are well ventilated. The other cockpit crew would be exposed to some amount of smoke. The amount that makes it to the passengers from a single cigarette in the cockpit would probably be undetectable.
Smoke detectors are not very effective at detecting cigarette smoke. I believe this is because most of the smoke passes through filters (the fairly ineffective one on the end of the cigarette and the two very effective ones in the smoker’s chest) which remove particulates and it is therefore not very dense. It takes pretty thick cigarette smoke to set one off.
I don’t know about the rules in Bangladesh, but most of the time airlines are not allowed to listen to cockpit voice recordings. In the US that would be a violation of eavesdropping statutes. The US law stipulates that they may only be used for accident investigation purposes and only transcripts of relevant portions may be released publicly. They can be subpoenaed for a trial only if the publicly released transcript is insufficient and

discovery of cockpit voice recorder recordings is necessary to provide the party with sufficient information for the party to receive a fair trial.
ISBA Amendments of 1990, 104 Stat. 4655. 

The pilot’s unions are very much against using the recording or transcripts for any other purpose than accident investigation.
It would end up being down to the other crew reporting them. I doubt it occurs often enough for anyone to keep track of the number of reports for it, and i doubt it would be reportable to any aviation authorities. It would be handled internally to the specific airline.