Any pictures by chance? Thats a new one for me.

Any chance it's a "replica" ?

Can you share some photos? It would be rare to see rusting on a stainless steel putter like the phantom 5.5. Maybe if the putter was wet and in a putter cover for a significant time (months) some small areas of surface rust "could" appear.

No mention of where it was purchased. Was it purchased from a Titleist account, on eBay. See issues below, that can cause rusting on golf clubs. If it was purchased thru a Titleist account (golf shop, retail store, etc) take it back to that store with your sales receipt and have them contact Titleist customer service.

Cleaning and Care for Titleist Golf Clubs
Titleist drivers, woods, hybrids, irons and wedges can be cleaned with mild soap and water and then dried with a soft cotton cloth. We recommend your golf clubs and headcover be completely dry before storage. Storing golf clubs in wet headcovers can lead to rust and corrosion and may not be covered by the warranty.

Cleaning and Care for Carbon Steel Scotty Cameron Putters
The unique feel of certain Scotty Cameron by Titleist putter models results from the use of milled carbon steel heads. This material is susceptible to oxidation and rusting if left damp. To preserve the natural finish of these putter models, you are urged to follow a few simple guidelines:

• Always dry your putter at the end of the day. We also suggest drying the putter a few times during the round.
• Always use a dry headcover.
• Never leave your putter in damp or cold atmospheres.

The Gun Blue, Oil Can and Teryllium series putters are most susceptible to oxidation so please follow these guidelines carefully. The Pro Platinum finish is more durable due to the plating between the soft carbon steel and the elements, but care should be taken to avoid dings that can rust.

Contact Titleist Golf Clubs Warranty and Repairs
The Titleist Golf Clubs Warranty and Repairs Department may be reached at 800-742-7609.