How to deal with "Noticing Stuff" rolls
No matter how they are included in the game as a mechanic, there is the matter of how they are to be used during play. This is where some different schools of belief come in to play and several techniques can be used. Most of them have plusses and minuses, since no form is perfect. I am going to do my best to cover a few of them and perhaps help you learn to handle them better in your game.
If you want there to be more player control and direct connection to events it is best to leave the choice of using the skill/ability up to the players, often with some strong hinting from he GM I am sure. This gives the players more control (or at least the feeling of more control, depending on the GM runs things behind the scenes) over when they make discoveries and find things. This is best used for special search related skills, like those used to find secret doors or hidden items, but can be used for nearly any kind of spot check.
If you want the players to feel as if they are simply in the environment and all noticed things are just part of the description, it is best to have the GM make all the rolls (often ahead of time) and only tell the players things based on what they noticed. Further rolls could possibly be made if they feel the need to examine or search something further, but these would not be the standard Spot Checks used in most cases, often requiring the use of specialized skills.
Most of the time you will find the right balance being somewhere in the middle of these two extremes, or even a mix of the two. I like to allow the players to announce when they are going to further examine something, but base what I tell them off secret rolls made ahead of time (or even not rolling at all and basing it of their base score alone, if it's less important information). Again, the balance you choose should be based on the style of game you play and feel you are going for.
The most obvious way is to make the rolls in secret and not tell the players anything. When they find something you simply make it part of what you tell them in descriptions and when they don't you leave it out of the descriptions. In variable level systems, you tell them as much information as what the roll would normally have allowed, but without telling them anything about a roll having been made. Not all players like this, as they want to make their own rolls (or because they may even get bonus points, experience, or improvement based on use in some systems). This form also means more work on the GMs part, as they have to make and keep track of all these rolls.
There are also good arguments as to why the GM would want to keep any results rolled secret (even those make ahead of time). The main reason being misinformation due to horrible rolls can be given, but if the player made the roll, they will know it is wrong. This is the "fumble factor" on noticing. A fumble could lead to a character noticing something that isn't there, like seeing movement in the brush along the side of the road ahead (like an ambush) when there is nothing there. This could lead to them reacting and taking actions based on this misinformation, at least until they realize they were mistaken, which can be a fun little addition to any game.
Another way is to have a series of points, both of minor and major interest, mapped out in the story ahead of time (not all of which may be reached by the PCs) and have the players make a number of rolls before the play session (or even scene) begins. The GM keeps track of these rolls, success or not, and works their results into the descriptions and events of how the session plays out. This allow the player to feel the importance of and make the rolls, but doesn't give away when they are important to the plot so they don't react inappropriately at specific points. This still requires a little extra work on the GMs part, but gives a little more open feel to the game.
The other fun way to handle it is to just make your players roll for "noticing stuff" all the time (even for the most mundane things like a smell of salt in the air of the shore or a rock on the ground that is sorta shaped like a dog's head in profile). Most of the time these rolls will reveal unimportant things, so they simply become desensitized to reacting when asked to make them, but it allows the GM to ask for them to be rolled when they actually need them without risk. This takes very little extra work on the GMs part, but can become a little annoying to the players over the long run. Personally, I find it amusing to come up with pointless things to discover for all the minor ones.