The only thing you can tell for definite about a company that doesn't give an indication of salary in an ad is that for some reason they think their recruitment campaign is better in some way if they don't give that information.
-It could be that they genuinely have absolutely no idea what they'll pay the person, and are really that flexible about requirements and salary. Usually for more specialist roles, and the rest of the ad will usually indicate that as well if that's the case.
-It could be that they are concerned about the competition finding out what they pay staff, although giving a wider range usually addresses that.
-It could be that advertising a salary, or range, will give them issues internally with people who are paid less for similar work. In which case they don't have a fair salary system and I'd consider bypassing them anyway.
-It could be that they are inexperienced with recruitment and genuinely think saying something like that is more attractive to candidates even if the salary they are offering is perfectly decent.
-Or it could be that the salary or range they are considering isn't particularly attractive and they don't want to put people off applying.
The latter is the most common IME, and, unfortunately for employers who are doing it for any of the other reasons, most potential candidates will assume the latter is the reason and make their decision whether to apply on that basis.