The Porch is a room.
The key is a thing.
The mat is a supporter in the Porch. It is enterable. A mat can be covering.
Instead of looking under the mat:
if the player is on the mat:
say "You can't look under the mat while you're standing on it.";
otherwise if the mat is covering:
now the mat is not covering;
now the player carries the key; [move the key to the player;] [doesn't change anything.]
say "You lift a corner of the mat and find the key."
Produces these results …
Porch
You can see a mat here.
>Get on the mat.
You get onto the mat.
>Look under the mat.
You can’t look under the mat while you’re standing on it.
>Get off the mat.
You get off the mat.
Porch
You can see a mat here.
>Look under the mat.
>Inventory
You are carrying nothing.
It looks like you’ve stated that the mat can be covering, but not that it is covering. And the “Instead of looking under the mat” rule doesn’t say what to do if the mat is not covering. (I haven’t tested this.)
The Porch is a room.
The key is a thing.
The mat is a supporter in the Porch. It is enterable.
Check looking under the mat:
if the player is on the mat:
say "You can't look under the mat while you're standing on it." instead.
Carry out looking under the mat for the first time:
now the player carries the key.
After looking under the mat:
say "[one of]You lift a corner of the mat and find the key.[or]There's nothing else under there.[stopping]"
It looks like you’ve stated that the mat can be covering, but not that it is covering. And the “Instead of looking under the mat” rule doesn’t say what to do if the mat is not covering.
Instead rules can be so tricky and can have unpredictable(?) results because we tend to assume the player can actually ‘do’ the action described without considering possible limitations which may not be all that obvious. I’ve had so many laughable results with Instead rules.
Instead of hitting something with something:
say "You strike [the noun] with [the second noun] so many times, your bones vibrate for a few minutes."
hit me with ceiling fan
You strike the yourself with the ceiling fan so many times, your bones vibrate for a few minutes.
Always best to have an action go through its Checks or have Check rules in place for unique results. I find it’s best to have an Instead rule either only ‘say’ something, only ‘try’ something else, or both, unless it’s absolutely clear that there are no limitations blocking the action.