Bangladesh and India have been presented with a nice opportunity to cooperate in an area that will benefit India, as well as Bangladesh. Delhi should seize this opportunity to open a genuine dialogue with Dhaka on border issues. Although there are a number of outstanding disagreements, the Chittagong Hill Tracts (like the Rajshahi border) offer an area where the two can cooperate in the interests of common defense and shared border security.
Recently, five Indian nationals were arrested in the Noymail area of Dighinala sub-district, Khagrachhari. They evidently entered the country through Tripura, (which makes geographic sense) and Bangladeshi security forces believe they are members of the National Democratic Front of Bodoland (Daily Star).
The NDFB, seems to be composed of two factions, one is engaged in talks with the government while the other is blamed on a string of violence since at least last year which has left scores dead and hundreds injured. Both are working for a separate homeland for the Bodo tribe in India's often neglected northeast (which include the states of Mizoram, Assam, Meghalaya, Manipur, Nagaland, and Arunachal Pradesh). A number of these states are experiencing various levels of unrest and separatism which mostly stem from tribal-nationalism, uneven regional development, and anti-migrant sentiments (there are a large number of Bangladeshis in Assam).
In March, the NDFB was blamed on a bombing in which 24 were injured while at market in northeastern Assam (BBC). A few months later, Indian intelligence agencies reported that the rebel-NDFB faction was planning bomb attacks throughout Assam (Telegraph).
These arrests seem to suggest that the group is quite a distance away from its normal range. The Bangladeshi security services will no doubt attempt to find out why they crossed into the CHT. Dhaka should provide any relevant information to India as well to address any upcoming threats; Delhi should reciprocate and provide Dhaka with any details regarding the reasons the NDFB would be in the CHT in the first place.