BillGuard is pairing its transaction monitoring technology with an early-stage Silicon Valley company, Lemon, that is already giving smartphone users a way to track the cards and receipts in their pockets.

The New York startup has been working with the digital wallet outfit since last month, says BillGuard's founder and chief executive Yaron Samid.

The partnership is providing roughly 2.5 million Lemon Wallet customers the ability to catch fraudulent purchases on their cards.

Lemon lets its users to add digital card accounts into its wallet by snapping pictures of their physical cards. Lemon then recognizes the issuer and pairs the card with online information that a bank or credit card company already provides.

The pact is pushing BillGuard closer towards its ultimate goal of creating a national database that will accurately flag dubious charges that cardholders might not otherwise notice.

Earlier this fall, BillGuard added an integration into Apple's Passbook — a closed loop digital wallet that allows iPhone users to make payments using barcodes — that performs a similar function.

"We have put this together pretty quickly in a manner of a few weeks," said Samid, in an interview with American Banker.

"With Passbook and Lemon we are going to do this in a way where we are leveraging the fact that you can plug BillGuard  into a bank, into a digital wallet, and it works beautifully," he said.

Fraud monitoring services such as BillGuard's are answering concerns over security, said Rick Oglesby, a senior analyst at the Aite Group, in a press release.

"Digital wallets are getting a lot of attention — especially as major brands like Apple and Google continue to unveil new offerings," he said. "There is still legitimate concern over security and even concerns about over-spending due to ease and convenience."