Just looking in my handbook it shows 2 master keys, one without remote buttons, and says dealer can use either.
It also shows a metal tag with a number which can also be used to make duplicate keys.
Mind you, even if this is the case, it'll still be no giveaway from a dealer.
On an Accord tourer I owned, I bought a full used key and a new blank, combined the old innards with the new blank, got the blade cut then took the key to Honda for programming to match my car.
The mobile locksmith who cut the key reckoned he'd be able to program it, having done other makes on his laptop, but failed, so had no choice but to get bent over by Honda.
Whether you can do that with Lexus keys I don't know, but it saved me about £50 on the Honda.