On the other hand, if it's a great old one you really don't want them to notice you. Think "if they notice I'm siphoning power from them, it could trigger a minor apocalypse". Think more along the lines of you're getting power from Cthullu.
That's the typical GOO. The range which is available, within the RAW for GOO, includes other options. I DMed for a player who had not really thought much about his Patron. I decided that the Patron was a 17th-dimension intelligence, doing something parallel to a graduate student's research on fruit flies, trying to understand them better, by making direct contact with one of them. The short lives of creatures limited to only the first 3 dimensions was a fascinating topic for the Patron, who never fully understood what eating is all about, nor roads. (I mean, why not just use the 9th-dimension shortcut... oh, right, I guess you can't. So you combine bricks and wood to make roads? Are sheep and wheat involved? Why not ore?)
Intelligence way beyond ours, check. Techniques not available to most mortals, check. Bizarre, check. Hostile, not at all - though occasionally careless. (You realize how flammable ants are, when you examine them with a magnifying glass under bright sunlight.) Most importantly: fun for the DM, for the player, and for the other players overhearing the warlock-patron conversations.
My favorite scene happened when the PCs captured an NPC cultist of Tiamat. Tiamat granted the cultist another Warlock level and an Invocation suitable for escaping. The PC's patron, however, 'eavesdropped" on the conversation between Tiamat and cultist, woke up the PC, imparted the "intercepted" Invocation to the PC, and then the PC interrupted the Tiamat cultist's escape attempt. It took the PC a while to realize that "Entity 2389" was his Patron's way of referring to Tiamat, and that the cultist was a warlock (the cultist had not revealed any magic during or after the capture).
Obviously the real answer is what makes sense for your world and what you think will be most fun. Don't fall into the trap of trying to force your players to play the way you "think" they should. For some people, background fluff is irrelevant to their enjoyment of the game. So encourage and reward, but don't badger or punish if they aren't playing the "right" way.
And there, Oofta, is where I agree with you, 100%, word for word.