Hugo Kornelis gives us a great deal of information on index seeks in SQL Server:
Every Seek Keys specification can be either for a “singleton seek”, or for a “range seek”. A singleton seek applies when at most a single row can satisfy the requirement of the Seek Keys specification. A range seek means that (potentially) more than a single row can qualify.
For a singleton seek, the index structure is used to find the row that matches the specified condition. If it exists, it is returned and then the operator immediately continues to the next Seek Keys specification. If it doesn’t, then nothing is returned and the operator continues to the next Seek Keys specification.
Read the whole thing and pair it with index scans if you haven’t read that already.