My wife has been going through batteries in her Keyless-Go fob at an alarming rate.
I either got a pack of bad batteries, but we switched fobs and I am doing OK, and she is still having the problem.

One of two things is going on. Something in her car is polling the key more often then usual and wearing out the battery. (since she drives that car more than I do)
or,
Her Samsung Phone with NFC communications (in the same purse as the key 99% of the time) is falsing the key into transmitting and wearing out the batteries.

Thoughts?

I'm kind of stating the obvious, but
have her turn off the NFC function on the phone for a while and see how it goes. She can always turn it back on in about 2 clicks when she NEEDS it on.

That is on the list. She does not use NFC, but updates seem to turn it back on.

Just wondering why she is always having and issue and I am not.
We have even swapped keys.

NFC frequency = 13.56 MHz
Keyless go frequency in the USA - 315mhz

They key is probably faulty. The red indicator lights when the key is being polled, but I have replaced a few keyless keys that drain batteries quickly due a fault on the key.

I first figured it was a bad battery (or bad pack of batteries, since it has happened more than once).
What put me looking for another issue is we swapped keys, and a few days later she started having trouble again.

The Keyless Go uses a VLF frequency for key location and identification, and 315MHz for normal remote operation.

Definitely the phone. I have an iphone for work and an android for personal. When I do carry the iphone, I keep it in my key pocket (personal is always in a separate pocket to avoid key scratches). The car often doesn't register the key when the iphone is sharing the pocket, implying there's definitely interference. Never happens when the iphone is gone. I also blow through batteries, but they're cheap and I don't really care.

OK, since someone else has a similar thing going, I am going to shut off NFC and see if it helps.

The other thing is contactless charging, as I think that also uses VLF frequencies.
Not that key is near the charger, but maybe the phone's checking for a charger and the key interact?

Well, the car transmits on 125Khz, and when the key detects this it sends the data back on 315mhz.

I agree turn off bluetooth/nfc etc and see if it still happens

Just checked my wife's phone, and NFC was off. (darn it).

Where does she the leave her key fob in the house? My wife takes the primary spot in the garage next to the door to the kitchen. She puts her purse down on the table, which is about 13 ft from the centerline of her ML350. I get the servant's spot in the garage, which is approx 30 ft from where I leave the key fob for my GL450. I've seen it mentioned here before that the keys talk to the cars and vice versa all the time if they are in proximity to one another. I have to change the batteries in both ML key fobs about every 6 months. I've yet to change the GL battery after 6 mos of ownership.

Her fob is almost always in her purse (as is her phone 90% of the time), About 40 to 50 feet form the car when in the house.
My fobs sit in a drawer about another 20 feet.

She isnt inadvertently pressing the buttons in her purse and she walks along?