Plenty of women have historically used bras as a storage compartment of sort, for cash, receipts, and even keys. And it's been generally done to secure rather than to conceal for nefarious purpose. If you have no secure pockets on your (non-undergarment) indoor clothes and/or want a secure place to keep a valued item, placing a valued item in the bra may be done for reasons that have nothing to do with nefarious intent.