I have a single entry working visa for Thailand. I live and work in Thailand. Can I easily leave Thailand without problems regarding my Visa? Or will there be problems at the airport at emmigration? With leaving, I mean within 1 day, without letting my employer know. Or is it actually required to let my employer know beforehand?
First a few things regarding terminology.
There is no such thing as a "work visa". There is a "non-immigrant B" visa which is issued by a Thai embassy overseas to allow people to enter Thailand for business purposes. These can be single-entry or multiple-entry. If you enter on one of these visas, you will get a stamp allowing you to stay in Thailand for 90 days from the time you enter the country. However, you are not allowed to 'work' using this visa.
Then there is a "work permit", which is issued by the Department of Labour within Thailand to allow someone to actually work in Thailand. This is a separate blue book with details of your employer, the place you are allowed to work, and a photo of you along with your details. This is required if you are working in Thailand.
If you are working long-term with a work permit, you will usually get an "extension of stay" in your passport that allows you to stay up to a year (or two years in some cases).
Finally, there is a "re-entry permit", that allows you to leave Thailand once (or multiple times) and come back and it does not affect your original "allowed to stay" date. This can be used to enter Thailand several times on a single-entry visa within 90 days, for example.
Now to address the question that was asked.
Are you intending to leave Thailand and then come back again? Or are you intending to leave and never come back?
If you intend to leave and come back and you have a single-entry visa to Thailand, then you need to get a re-entry permit (see 4 above). You can get these at the immigration office at Chaengwattana, or at the airport.
If you intend to leave and come back and you have a work permit and an extension of stay, you also need to make sure you have a re-entry permit, or else your extension will be cancelled when you leave. Usually an employer will get a multiple-entry re-entry permit when they apply for the extension of stay, but if your employer is a cheapskate they may not have done this. Check in your passport.
If you intend to leave and never come back, then you just leave. Your employer will find out when you don't show for work, and they will try to contact you. After three days they can decide that you have deserted your job, and they will need to cancel your work permit (if you have one), which also cancels your extension of stay (if you have one). If you only have a visa (not an extension of stay), your employer can not cancel that, and you can stay in Thailand for the original 90 days they stamped in your passport.
IMPORTANT, if you don't have a work permit and you are working in Thailand, there are heavy penalties for this, and people who are caught working without a work permit are subject to a fine and deportation. If anyone asks you if you are working in Thailand, do not answer "yes" unless you have a work permit.
Finally, you don't have to tell your employer anything. Your only obligation to them is to keep within the contract you signed with them.