I don't know if you've noticed is a present tense, so that means the time period we're talking about hasn't finished. So if the person didn't notice in the past, they still have time to notice now - the thing that they hadn't noticed is still happening. I don't know if you've noticed, but the car park will be closed next Tuesday. I don't know if you've noticed, but the shop doesn't sell flowers anymore. I don't know if you noticed is a past tense so, strictly speaking, it refers to something that could have been noticed in the past but can't be now if it wasn't then. I don't know if you noticed, but he wasn't at the party. I don't know if you noticed, they moved out last week. In reality, many native speakers will use 'I don't know if you noticed' in both situations, but a careful speaker should chose the appropriate one.

Good question! As Adam says meaning-wise they are essentially the same. There are two slight differences, if we are getting into specifics... 1. The use of present perfect "I don't know if you've noticed" sounds open ended, almost like a question in the form of a statement (Did you notice? Maybe you did notice.), while the simple past (completed action) "I don't know if you noticed" sounds more definitive and final (I think you didn't notice.). 2. When describing past actions, British English speakers tend to use present perfect more frequently, whereas Americans tend to use past simple (except in cases where present perfect is required, e.g. "How many times have you seen that movie? I have seen that movie 5 times."). So, "I don’t know if you’ve noticed" sounds a little more British and "I don’t know if you noticed" sounds a little more American. Hope this helps!

The difference between "I don’t know if you’ve noticed" and "I don’t know if you noticed" is in the use of the contraction "you've" in the former and the absence of it in the latter. "I don’t know if you’ve noticed" uses the contraction "you've," which is a shortened form of "you have." This form implies that the speaker is referring to something that may have been noticed by the listener at some point in the past leading up to the present moment. "I don’t know if you noticed," on the other hand, does not use the contraction "you've." This form implies that the speaker is referring to something that the listener may or may not have noticed in the immediate or recent past leading up to the present moment. In practice, the difference between the two phrases is quite subtle, and they can be used interchangeably in many situations. The choice of which to use may depend on the context and the speaker's preference for using contractions or not.

The first ("you have") is present tense and the second ("you noticed") is past tense. That's the only difference, clear and simple. Do you want to talk about what the person's current his experiences are (his memories of the things he has noticed) or do you want to ask the person about the past (what he noticed)? In most cases, it makes little difference which one you use but in special situations where you want to talk about his past rather than is current memories, or vice versa, then you have only one choice. Do they mean exactly the same thing? No. If Bob is a dead person you cannot say "I don't know if Bob has noticed" but you can say "I don't know if Bob noticed". Both are grammatical but the first is nonsensical.

The only difference is the change in tense: past perfect vs simple past. Semantically they mean the same thing.