See a pin and pick it up, all the day you’ll have good luck;
See a pin and let it lay, you’ll have bad luck all the day.

I don’t know about luck coming into it, but pins can be dangerous in the wrong place.  As for the second line, rhyme takes precedence over grammar.  It should be “let it lie”.  Hens lay (eggs).

It’s like looking for a needle in a haystack.
This is a very popular expression, but how much thought do people give to its origins?  Hay and needles are long, thin and sharp at the end.  Nowadays haystacks are not a common sight.  When I was a child the hay was collected into sheaves and piled up into a haystack.  A haystack was roughly the size of a shed. So it is very unlikely that the (hypothetical) needle would be found.

Stacked hay in Romania Photo credit Wikipedia Originator Myrabella

The devil will put it back when he’s finished with it.
Have you ever thought about how much time is spent looking for things?

Mothers are usually much better at finding things than their children.  Here are three of the things they are likely to say to them before or after a search.

Use your eyes!

You can’t see for looking.

You can’t have looked properly. 

By contrast someone who is very observant is said to be eagle-eyed.

What have you got to lose?
Encouragement to someone to make a decision to do something.

Leave no stone unturned.
Have you ever lifted a stone and been amazed at the creatures living under it?  This saying is to do with opportunity rather than looking for something.

X marks the spot.
On a map or plan a cross is used to indicate the position of something.  Spot here is place and nothing to do with spotting or noticing something.

We’ll see how the land lies.
Builders and architects look at the land to see how best to build on it.  The expression has come to mean seeing how people react.

We shall see.
A comment about waiting to see what happens in the future.

A place for everything, and everything in its place.
There was a children’s story about some goats who never lost anything because they lived by this motto.  According to The Oxford Dictionary of Phrase, Saying and Quotation Mrs Beeton used it in The Book of Household Management and it is sometimes attributed to Samuel Smiles.

Shipshape and Bristol fashion
is a nautical term for being in good order.

Look before you leap
is a proverb about considering the outcome before you take action.

Don’t stare…
…it’s rude.

Don’t look now…
…means what it says, but can just be an expression.

It gave itself up.
Something was lost end then found by chance.  The analogy is with a miscreant giving himself up to the police.

Finders keepers, losers weepers.
This implies that if you find something you are entitled to keep it and the person who lost it may be upset.  Sometimes there is a reward for doing the honest thing.

Jesus told a story about a man who found a great treasure.  Matthew Chapter 13 verse 44 (NIV) The kingdom of heaven is like treasure hidden in a field.  When a man found it, he hid it again, and then in his joy went and sold all he had and bought that field.