Perception. Wisdom (Perception) check lets you spot, hear, or otherwise detect the presence of something. It measures your general awareness of your surroundings and the keenness of your senses. For example, you might try to hear a conversation through a closed door, eavesdrop under an open window, or hear monsters moving stealthily in the forest. Or you might try to spot things that are obscured or easy to miss, whether they are orcs lying in ambush on a road, thugs hiding in the shadows of an alley, or candlelight under a closed secret door.
Investigation. When you look around for clues and make deductions based on those clues, you make an Intelligence (Investigation) check. You might deduce the location of a hidden object, discern from the appearance of a wound what kind of weapon dealt it, or determine the weakest point in a tunnel that could cause it to collapse. Poring through ancient scrolls in search of a hidden fragment of knowledge might also call for an Intelligence (Investigation) check.
The way I view it is that Perception is about knowing that something exists. Doesn't tell you anything about it other than the fact it is there and whatever information your sense gives you.
Investigation is about deducing a larger picture from the things you perceive. In GURPS Perception would be coupled with a specific knowledge skill. For example you use perception to find a shiny golden rock and geology to tell whether it is fools gold or real gold. In 5e Investigation is what would be used in place of the Geology skill.
I would recommend limiting Investigations to deducing any information on specific objects. Leave it to player skill to figure out the larger picture that connects the details that are uncovered. It very easy for a referee in a crime scene encounter to allow his player a Sherlock Holmes moment via a simple successful investigation check. I think this would take too much away from the experience of being there as the characters.
For example a party discovers Mr. Peabody dead in the foyer. Through a successful series of Perception, Medicine, Wilderness Survival and Investigation checks. They find
- Mr. Peabody was killed by a blunt instrument (Medicine)
- The instrument was a candlestick (Investigation)
- The candlestick was in the bushes (Perception)
- A bloody footprint in the mud nearby (Survival)
- The footprint was of a man of medium height and weight (Investigation)
- They met earlier Miss White a older heavyset women.
- Met earlier Colonel Mustard a middle aged man of medium height and weight
- Met earlier Miss Black a young petite women in her 20s.
- Met earlier Professor Plum a middle aged man of medium height and rather thin.
I would leave it up to the players to put this information together to deduce that it was Colonel Mustard in the foyer with the candlestick who killed Mr. Peabody rather than have them make another investigation check.