Okay, time to sit Grandma down, and nicely, lay out the rules.
This is your child.
I have relatives, that take my kids out. They are now 5 and 9.
I ALWAYS, tell them, to CALL me and let me know, where they will be going... if it was not pre-planned or spur of the moment.
They do. And certainly, they do NOT know... of every little parental preference we have, about my kids. And some of these relatives, do not have kids of their own, so their perception, of what to do or not with a child... is different.
Now... on the other hand: Ever since my kids were 2 years old... I have taught my kids that:
1) WE are the parents. Not anyone else.
2) THEY can speak up and tell Grandma or Aunty or relatives "no" or "I don't want to..." or "I have to ask Mommy first." And my kids do. Even when they were only 3 years old they spoke up.
3) I teach my kids, about the Totem-Pole of adults in their life. And the pecking order, so to speak. Mommy & Daddy, decide things for them... they know what is right/wrong, allowed or not, per us. And they need to SPEAK UP, if something is not comfortable for them or they just don't want to go somewhere or do something that a relative is telling them about. And my kids do, speak up.
4) You have to be pro-active and ALSO teach your child... about things. So that, THEY can as they get older, "discern" things too, per your parental ways. Not just relying on the relative, to decide or the relative doing whatever... with your child, without your permission.
5) There have been many times, my kids have been on outings with Grandma or Aunty or other relatives.... I will get a phone call from them (because my kids will TELL the relative "I want to call Mommy, can I use your cell phone... and ask Mommy first..." and they call me to say "Aunty wants to take us to her hair salon, can we?" or "Aunty said we can go and get a pie to eat at her house... can we?" My kids speak up. But it takes teaching that... to your child. I also teach my kids... that if they are with relatives or friends and they are tired and want to come home, they NEED to say so and it is fine. And that, if they do NOT want to do something with a relative, they can also SAY that. It is good. They need to speak up. And they do.
So in the realm of things and as your child gets older or even now. Teach him... these things and how to speak up and that Mommy/Daddy are the parents.
I would be real irked... if any of my relatives... took my kids 50 miles away to another city... or 2 hours away to a beach WITHOUT telling me, nor getting my permission, first.
That, is unacceptable.
And if she could not get a hold of you, then she must KEEP trying... or not go. At all.
DO NOT let Grandma do these things.
That is not her child.
You need to lay down the rules. Do not be shy about it. And teach your child... about how to speak up and know things too.
Even my 5 year old son, will tell his Aunty "I don't think Mommy would want us to go there..." or he will tell her "I am tired, I want to go home."
Because, Aunty has a tendency to be on the road for HOURS at a time, with my kids, on an outing, and get sidetracked.
And by the way, having "quality time with Grandma"... does NOT mean, going 50 miles away to another city on a train. They can have quality time, even at home, playing.
You need to STATE the rule and boundaries, with Grandma.
Or the limits will keep... getting stretched. If you do not say anything.
As it is... Grandma is taking your child all over the place and you don't even know. They aren't even home... when you go to pick him up! And the other time, your Step-Dad didn't even know where they were!
Good grief.
Being a good Grandma or being interested in and loving a grandchild.... does not mean, having carte blanche over the child. It also takes being responsible and informing the parent, of things. And letting a Grandparent be a nice Grandparent, also does not mean that the parent cannot lay out rules, about their own child.