Non-invitation to friend's retirement party

My best friend is retiring at the end of this month. We worked at the same agency but at different locations. Her current co-workers would not know me. I retired two years ago. Her office is having a retirement party for her. I received a copy of the invitational email from a work-friend of a work-friend. When speaking with my best friend, I nonchalently asked if her office is doing anything. She did tell me about the party but did not provide specifics, nor said anything about my attending...didn't ask if I was going...or would I like to go. I see the initial email about her party was addressed to some acquaintances and friends she and I know. She's very popular and I assume many of them will go. I want to speak with other friends in my situation, but that's not polite. My instincts tell me I should not pursue this any further...just send her a card and wish her well. I don't want her to be mad or hurt...but mostly I don't want her to feel obligated to include me. Yet, those friends who know me, as well, will assume I will be there and when I don't show, I look like the snubber! What should I do? And if, by chance, my best friend asks for an explanation, what do I say to her? Thank you in advance.