When I got down at Mumbai, my bag was marked 'X'. A few other peoples' bags were also similarly marked. What does it mean actually ?
I had to tell them what I had in my bag and also open and show them but others were not as thoroughly searched ! Any idea ?
Customs officers everywhere cannot practically check everybody who passes through; they choose who to search on a mixture of 'information received' and 'random searches'. What the mixture is, and exactly how random the latter are, are closely-guarded secrets (as you'd expect); but I would guess the chalk was simply marking which passengers had been randomly chosen for a full inspection.
In some countries/airports, some or all of the bags may go through scanners before being delivered on the belts.
If the scan reveals anything suspicious they may mark the bag for customs officers to verify the contents in your presence (scanners are not foolproof, so there may be false positives where they flag your bag for inspection even if there is nothing problematic).
Depending on the procedures (and what has been detected), this may involve marking the bag, or just advising customs officers if they want to be able to monitor you without raising your suspicion.
