Rats carry disease and are an increasing pest problem in the UK. Like mice rats reproduce very quickly meaning any infestation should be treated urgently.
Rats are usually nocturnal creatures so any spotted in daylight would indicate greater numbers at night.
Gnawing, strewn food rubbish, droppings and holes in the garden are all signs of rats.
Suggested positioning of traps:
Avoid open areas, rats are creatures of habit so look for signs of rat activity (droppings, urine etc)
Chocolate, sweet smelling food, cereal, dog biscuits