Learn English – Wind an electric car window down

In agreement with talrnu’s comment, I think “window shopper” is in fact appropriate here. In usage (which might be better represented on Urban Dictionary and Yahoo Answers), this term is commonly employed to refer to people who walk around both inside and outside of shops, and is used to differentiate such people from those who have more of an inclination to make a purchase.

From Urban Dictionary:

1. Someone who looks at stuff they can’t buy.

2. When one visits a store or mall to admire goods rather than to purchase them.

From Yahoo Answers:

It implies [. . .] someone who is not serious (a shopper who has no real intention of buying [. . .])

“Browser” does not sound right to my ear as it seems like an awkward nounification. Most people will think of something they use to view websites. I have not heard the word used this way. It would probably require explanation, which would defeat the purpose of having a single word for it.

That said, I have heard people say that they are “just browsing” to indicate that they are not actively pursuing a purchase at that moment. For your case, I could certainly see someone saying “he’s just browsing.” As far as saying “he’s not a customer”, you have another problem.

Anyone visiting the store is, potentially speaking, a customer. so it’s not ideal to say “he’s not a customer” unless you are pointing to an employee, security guard, pomeranian, etc. You might be clearer if you qualify the word “customer” before you establish this comparison, for example: “he’s not a serious customer, he’s _____________.”

Lastly, another term I know for such a person is to describe them as a looky-loo.

looky-loo

(also lookie-loo)
NOUN
^INFORMAL

1.1 A person who seems interested in making a purchase, but whose actual intention is only to browse:

a treat for all the North Shore’s looky-loos: the popular Spring Designer Kitchen Tour

A good business broker will separate the real buyers from the looky-loos, bring in more qualified prospects, and usually can garner a better price for the business.

_{Source: Definition of looky-loo in Oxford Dictionaries}

At first I thought it might have been an expletive that was deleted.

If the sentence had been

Your car has been bleeping stolen

I would have interpreted that as a placeholder for a swear word.

But in context it is clear he is talking about a car remote keyless system. Typically these make a bleeping sound indicating that the door was locked/unlocked successfully.