TLDR; they should already know that you are probably looking without you having to explicitly state it.

If your unhappy with your current job then they should know that by now. If you've already expressed what you are looking for in a job, and how you feel your current position doesn't fulfill that need, then they know you are dissatisfied with your current position. If you feel you are not progressing towards what you would like out of your job they should know that by now.

If they don't know that by now, then you haven't communicated clearly enough with your employers to try to make the most of your current employment. Instead of simply moving on to try to "fix it" yourself, you need to make it clear what the issue is first. If they don't know the problem, they can't solve it (or even try).

If they DO know the issues you have with your current employment, and they haven't been able to address it, then they know (at least in the back of their minds) that they are at risk of losing you.

By explicitly stating that you are looking for another job in essence expresses to them, that you've completely given up on reaching your goals at their company. As a result, they will give up on you and you then have no other choice but to find another job.

TLDR; they should already know that you are probably looking without you having to explicitly state it.

If you're unhappy with your current job then they should know that by now. If you've already expressed what you are looking for in a job, and how you feel your current position doesn't fulfill that need, then they know you are dissatisfied with your current position. If you feel you are not progressing towards what you would like out of your job they should know that by now.

If they don't know that by now, then you haven't communicated clearly enough with your employers to try to make the most of your current employment. Instead of simply moving on to try to "fix it" yourself, you need to make it clear what the issue is first. If they don't know the problem, they can't solve it (or even try).

If they DO know the issues you have with your current employment, and they haven't been able to address it, then they know (at least in the back of their minds) that they are at risk of losing you.

By explicitly stating that you are looking for another job in essence expresses to them, that you've completely given up on reaching your goals at their company. As a result, they will give up on you and you then have no other choice but to find another job.