Let's just also say the 18 year old would have very reasonable grounds to believe the 14 year old was 15. Whatever you do, don't come out guns blazing, it will only make your daughter better at keeping secrets.
I know it must be very stressful for you but the most important thing is to keep the lines of communication open between you and your daughter.
I would also be speaking to them about the pill, and safe sexual practice.
Being on the pill does not remove the need to use condoms to prevent STIs - something a lot of people over look.
For me because I have a DD I automatically think girl - my answer remains the same if it was a 14yr DS dating an 18yr old girl or a same sex couple for that matter. I'd speak to the child and explain that I snooped because I am their mother and I care about them.
The fact they are sneaking around, means they know it is wrong anyway. Being 14, I'm wondering where they would socialise? How much time would they likely be able to do socialising 'alone' - as in when other adults are present?
I mainly ask as if they are reluctant to speak to you about the boyfriend they surely are not expecting to have them over to their home or go to their home?
Are they members of the same sporting club or something?
When are they expecting to actually see the boyfriend or is the relationship mainly via social media?
What would you do if your 14 year old was dating an 18 year old?
Let's assume you knew your child was dating someone, but you only found out their name and age by snooping.
Child refuses to speak to you as you're 'uncool'.
Admitting to snooping means child will find another means of communication with their special friend and it might be harder for you to keep tabs on what was going on.